The use of adjustable beds has been known for sometime. Prior art adjustable beds were generally used in hospitals and convalescent homes and were adjusted by mechanical levers or cranks. Later, the mechanical devices were replaced by motors which drove the adjustable beds into the desired position through the use of gear trains, chain and sprocket drives, or threaded shafts. Many of these motor driven beds were equipped with limit switches to prevent the excessive raising or lowering thereof.
Several examples showing articulated adjustable beds which utilize limit switches may be found in the prior art including:
U.S. Pat. No. 2,349,701 by J. W. Buttikofer et al.;
U.S. Pat. No. 2,605,481 by A. L. Burkhart;
U.S. Pat. No. 2,714,092 by F. W. Marshall;
U.S. Pat. No. 2,747,203 by C. E. Dawson;
U.S. Pat. No. 2,807,174 by H. D. Helsel;
U.S. Pat. No. 2,837,751 by S. M. McCall;
U.S. Pat. No. 2,913,738 by C. W. Wise;
U.S. Pat. No. 3,051,965 by J. C. Szemplak et al.; and
U.S. Pat. No. 3,300,794 by H. Altorfer.
Each of these patents describes the use of a limit switch to prevent the burn out of a motor should the user of the bed attempt to exceed the physical limits of adjustment. None of the cited patents disclose a safety device which can sense the presence of a foreign object, such as a hand, under the bed and prevent the lowering of that bed.